Semiconductor device, method of manufacturing the same, and power module

ABSTRACT

A semiconductor device includes an n-type drain layer, an n-type base layer provided on the n-type drain layer, a p-type base layer and an n-type source layer partially formed in surface layer portions of the n-type base layer and the p-type base layer, respectively, a gate insulation film formed on a surface of the p-type base layer between the n-type source layer and the n-type base layer, a gate electrode formed on the gate insulation film facing the p-type base layer across the gate insulation film, a p-type column layer formed within the n-type base layer to extend from the p-type base layer toward the n-type drain layer, a depletion layer alleviation region arranged between the p-type column layer and the n-type drain layer and including first baryons converted to donors, a source electrode connected to the n-type source layer, and a drain electrode connected to the n-type drain layer.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Divisional of U.S. application Ser. No. 13/339,072, filed onDec. 28, 2011, and allowed on Sep. 3, 2014, the subject matter of whichis incorporated herein by reference. The parent application Ser. No.13/339,072 is based upon and claims the benefit of priority fromJapanese Patent Application No. 2010-292107, filed on Dec. 28, 2010, theentire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a semiconductor device having a MISFET(Metal-Insulator-Semiconductor Field-Effect-Transistor) of a superjunction structure and a method of manufacturing the same.

BACKGROUND

MISFETs used as switching elements in an inverter circuit or a powersupply circuit are largely categorized into a planar type MISFET and asuper junction type MISFET. The planar type MISFET includes, e.g., adrain layer, an n-type base layer arranged on the drain layer, a p-typebase layer formed in a surface layer portion thereof, an n⁺-type drainlayer and an n⁺-type source layer formed in a surface layer portion ofthe p-type base layer in a spaced-apart relationship with each other. Agate electrode is arranged to face a surface of the p-type base layerexisting between the n⁺-type source and the drain layer through a gateinsulation film.

On the other hand, as disclosed in International Publication No. WO2010/024433, the super junction type MISFET includes, in addition to theconfigurations of the planar type MISFET, a p-type column layerextending from the p-type base layer toward the drain layer. Thisstructure helps reduce on-resistance and increase switching speed. Oneof the problems posed in the super junction type MISFET resides in thata reverse recovery time trr of a parasitic diode is long. The parasiticdiode is formed by a p-n junction between the p-type base layer and thep-type column layer and the n-type base layer. In the super junctionstructure provided with the p-type column layer, a lot of carriers arestored in the p-type column layer. Therefore, when the parasitic diodeis turned off, a large reverse recovery current attributable to thecarriers flows for a relatively long period of time.

In a power module configured by serially connecting a high-sideswitching element and a low-side switching element, the reverse recoverycurrent induces a through-current. Thus, the reverse recovery currentflowing for a long period of time leads to an increased loss of energy.For example, if a large reverse recovery current flows through theparasitic diode of the low-side switching element while the high-sideswitching element is kept turned on, a large through-current is inducedand, therefore, a loss of energy is increased.

International Publication No. WO 2010/024433 provides a solution to thisproblem. More specifically, in the invention of InternationalPublication No. WO 2010/024433, a trap level (recombination level) islocally formed within an n-type base layer between a p-type column layerand an n⁺-type drain layer by performing baryon irradiation, therebyshortening the reverse recovery time trr. The trap level is formedlocally and is not formed in the p-n junction portion between the p-typebase layer and the p-type column layer and the n-type base layer.Accordingly, a leak current is not increased during an off-time period.

Another problem inherent in the super junction type MISFET is the hardrecovery of the parasitic diode. By the term “hard recovery,” it ismeant that a change of the reverse recovery current (di/dt) is fast. Inthe super junction type MISFET, depletion layers are expanded from notonly the p-type base layer but also the p-type column layer when theparasitic diode is turned off. In particular, the depletion layerexpanded from the p-type column layer rapidly joins the depletion layerexpanded from another p-type column layer adjoining thereto and rapidlyreaches the drain layer lying just below the depletion layer. For thatreason, the current is changed sharply and the reverse recovery currentis interrupted at a high speed. As a result, the reverse recoverycurrent shows a steeply-varying large-amplitude oscillatory waveform(ringing). These reverse recovery characteristics (hard recoverycharacteristics) generate a lot of noise and may possibly cause anerroneous operation of a controller for supplying a control signal tothe MISFET. In an inverter circuit, among others, for driving aninductive load such as an electric motor, the parasitic diode is turnedon and off. Thus, the hard recovery characteristics at the time ofturning off the parasitic diode become a problem.

The invention of International Publication No. 2010/024433 succeeds inshortening the reverse recovery time trr but fails to provide a solutionto the hard recovery problem. Accordingly, problems remain unsolved whenthe invention is applied to a power module (a power module for drivingan inductive load among others).

SUMMARY

The present disclosure provides some embodiments of a semiconductordevice having a super junction structure and capable of alleviating hardrecovery and a method of manufacturing the same.

According to one embodiment of the present disclosure, there is provideda semiconductor device, including: an n-type drain layer; an n-type baselayer provided on the n-type drain layer; a p-type base layer partiallyformed in a surface layer portion of the n-type base layer; an n-typesource layer partially formed in a surface layer portion of the p-typebase layer, a gate insulation film formed on a surface of the p-typebase layer between the n-type source layer and the n-type base layer; agate electrode formed on the gate insulation film to face the p-typebase layer between the n-type source layer and the n-type base layeracross the gate insulation film; a p-type column layer formed within then-type base layer to extend from the p-type base layer toward the n-typedrain layer; a depletion layer alleviation region arranged between thep-type column layer and the n-type drain layer within the n-type baselayer, the depletion layer alleviation region including first baryonsconverted to donors; a source electrode electrically connected to then-type source layer; and a drain electrode electrically connected to then-type drain layer.

In the semiconductor device, the p-type column layer extends toward then-type drain layer, thereby forming a MISFET having a super junctionstructure. If the drain electrode is connected to an electric potentialhigher than the source electrode and if a control voltage higher than athreshold voltage is applied to the gate electrode, an inversion layer(channel) is formed near the surface of the p-type base layer. Thus,there is formed a current path extending through the drain electrode,the n-type drain layer, the n-type base layer, the inversion layer onthe surface of the p-type base layer, the n-type source layer and thesource electrode in the named order. If the control current is notapplied to the gate electrode, the inversion layer is not formed and,therefore, the current path is blocked. The p-n junction between thep-type base layer and the n-type base layer and between the p-typecolumn layer and the n-type base layer makes up a parasitic diode. Aparasitic diode is kept in an on-state when a forward voltage is appliedand kept in an off-state when a reverse voltage is applied. If theparasitic diode is turned off, a reverse recovery phenomenon occurs thatcarriers (holes) within the p-type base layer and the p-type columnlayer migrate toward the source electrode while carriers (electrons)within the n-type base layer and the n-type drain layer migrate towardthe drain electrode. The current flowing at this time is a reverserecovery current. Due to the migration of the carriers, a depletionlayer expands from the p-n junction and the parasitic diode comes intothe off-state.

In the present disclosure, the depletion layer alleviation region isarranged between the p-type column layer and the n-type drain layerwithin the n-type base layer. The depletion layer alleviation regionincludes baryons converted to donors. The depletion layer alleviationregion restrains a depletion layer from expanding toward the drainelectrode, thereby reducing an expansion speed of the depletion layerwhen the parasitic diode is turned off. This reduces a changing speed ofthe reverse recovery current and, therefore, improves recoverycharacteristics. Accordingly, it is possible to provide a MISFET havinga super junction structure, which is good in on-resistance and switchingspeed and superior in recovery characteristics.

In another embodiment of the present disclosure, the semiconductordevice may further include a trap level region arranged within then-type base layer, the trap level region including second baryons forforming a trap level. With this configuration, the trap level regionincluding the second baryons for forming the trap level is formed withinthe n-type base layer. The carriers (electrons) migrating within then-type base layer are trapped in the trap level region, thereby reducingthe reverse recovery current. Accordingly, the recovery characteristicsare improved by the depletion layer alleviation region while the reverserecovery time is shortened by the trap level region.

In yet another embodiment of the present disclosure, the depletion layeralleviation region may have a thickness greater than a thickness of thetrap level region. If the trap level is widely distributed, a lot oftrap levels (recombination centers) are likely to be formed in the p-njunction portion between the p-type column layer and/or the p-type baselayer and the n-type base layer, and a leak current is likely toincrease. Accordingly, in some embodiments the trap levels are locallydistributed as far as possible within a limited range. On the otherhand, the depletion layer alleviation region is formed within the n-typebase layer over a relatively wide range in the area between the p-typecolumn layer and the drain layer, thereby effectively reducing theexpansion speed of the depletion layer and improving the recoverycharacteristics.

If relatively light baryons (e.g., protons) are irradiated on asemiconductor layer, the baryons entering the semiconductor layer aredistributed over a relatively wide range. In contrast, if relativelyheavy baryons (e.g., helium nuclei) are irradiated on the semiconductorlayer, the baryons entering the semiconductor layer are distributed overa relatively narrow range depending on the kinetic energy thereof.Accordingly, the depletion layer alleviation region can be made thickand the trap level region can be made thin, by selecting the relativelylight baryons as the first baryons for the formation of the depletionlayer alleviation region and selecting the relatively heavy baryons asthe second baryons for the formation of the trap level region.

In still another embodiment of the present disclosure, the secondbaryons may include one member selected from the group consisting ofprotons, ³He⁺⁺ and ⁴He⁺⁺. In still another embodiment of the presentdisclosure, the first baryons may include one member selected from thegroup consisting of protons, ³He⁺⁺ and ⁴He⁺⁺. In still anotherembodiment of the present disclosure, the first baryons may includeprotons. The conversion of protons to donors can be performed throughheat treatment at a relatively low temperature (e.g., at 350 to 450degrees Celsius) and, therefore, can be performed even after theformation of the source electrode or the drain electrode. Accordingly,the degree of freedom of the process gets higher. For example, if heliumnuclei (³He⁺⁺ or ⁴He⁺⁺) are used as the first baryons, the conversion todonors thereof requires heat treatment at a high temperature of about500 degrees Celsius. In order for the electrodes not to be damaged bythe heat treatment, it is therefore necessary to perform the heattreatment for the conversion to donors before the formation of theelectrodes.

In still another embodiment of the present disclosure, the depletionlayer alleviation region may include an area overlapping with the p-typecolumn layer. In still another embodiment of the present disclosure, thedepletion layer alleviation region may not include an area overlappingwith the p-type column layer. In still another embodiment of the presentdisclosure, the depletion layer alleviation region may adjoin the p-typecolumn layer (more particularly, the bottom surface of the p-type columnlayer). In still another embodiment of the present disclosure, thedepletion layer alleviation region may be separated from the p-typecolumn layer (more particularly, a bottom surface of the p-type columnlayer). In particular, in some embodiments a boundary of the depletionlayer alleviation region coincides with or remains close (by a distanceof, e.g., 10 μm or less) to the boundary of the p-type column layer.Accordingly, the speed at which the depletion layer expands from thep-type column layer can be reduced without eroding the p-type columnlayer.

In still another embodiment of the present disclosure, a power module,including: a high-side switching element formed of the semiconductordevice described above; and a low-side switching element formed of thesemiconductor device described above, the high-side switching elementand the low-side switching element serially connected to each other.With this configuration, the high-side switching element and thelow-side switching element are formed of the semiconductor deviceincluding the MISFET having a super junction structure. It is thereforepossible to provide a power module enjoying a reduced power loss in theswitching elements, thereby providing a power module enjoying increasedpower supply efficiency. In addition, the hard recovery occurring whenthe parasitic diodes of the switching elements are turned off isrestrained. It is therefore possible to reduce the erroneous operationof the controller for supplying a control signal to the switchingelements.

In still another embodiment of the present disclosure, the power modulemay further include three serial circuits in which the high-sideswitching element and the low-side switching element are seriallyconnected to each other, the three serial circuits parallel-connected toone another. This makes it possible to form a three-phase bridgeinverter circuit. Similarly, a single-phase bridge inverter circuit canbe formed by parallel-connecting two serial circuits in which thehigh-side switching element and the low-side switching element areserially connected. In addition, a single serial circuit (half-bridgecircuit) in which the high-side switching element and the low-sideswitching element are serially connected may be used as a unit of thepower module.

In still another embodiment of the present disclosure, the power modulemay be used to supply a drive current to an alternating current electricmotor. In an inverter circuit for driving an inductive load representedby an alternating current electric motor, the parasitic diode is turnedon and off. Accordingly, the load drive can be satisfactorily controlledif the semiconductor device capable of reducing the hard recovery of theparasitic diode is used as a switching element.

In still another embodiment of the present disclosure, there is provideda method of manufacturing a semiconductor device, including: forming ann-type base layer on an n-type drain layer; partially forming a p-typebase layer in a surface layer portion of the n-type base layer;partially forming an n-type source layer in a surface layer portion ofthe p-type base layer: forming a p-type column layer within the n-typebase layer, the p-type column layer extending from the p-type base layertoward the n-type drain layer, forming a gate insulation film on asurface of the p-type base layer between the n-type source layer and then-type base layer; forming a gate electrode on the gate insulation filmto face the p-type base layer between the n-type source layer and then-type base layer across the gate insulation film; forming a sourceelectrode electrically connected to the n-type source layer; forming adrain electrode electrically connected to the n-type drain layer; andforming a depletion layer alleviation region between the p-type columnlayer and the n-type drain layer within the n-type base layer byirradiating first baryons at a rear surface side of the drain layer andthen converting the first baryons to donors through heat treatment at apredetermined temperature.

In still another embodiment of the present disclosure, the method mayfurther include; forming a trap level region within the n-type baselayer by irradiating second baryons at the rear surface side of thedrain layer.

In still another embodiment of the present disclosure, the depletionlayer alleviation region and the trap level region may be formed suchthat the depletion layer alleviation region has a thickness greater thana thickness of the trap level region. As stated above, the depletionlayer alleviation region can be made thick and the trap level region canbe made thin, by selecting the relatively light baryons as the firstbaryons for the formation of the depletion layer alleviation region andselecting the relatively heavy baryons as the second baryons for theformation of the trap level region.

In still another embodiment of the present disclosure, the secondbaryons may include one member selected from the group consisting ofprotons, ³He⁺⁺ and ⁴He⁺⁺. In still another embodiment of the presentdisclosure, the first baryons may include one member selected from thegroup consisting of protons, ³He⁺⁺ and ⁴He⁺⁺. In still anotherembodiment of the present disclosure, the first baryons may includeprotons.

In still another embodiment of the present disclosure, the depletionlayer alleviation region may be formed to have an area overlapping withthe p-type column layer. In still another embodiment of the presentdisclosure, the depletion layer alleviation region may be formed not tohave an area overlapping with the p-type column layer. In still anotherembodiment of the present disclosure, the depletion layer alleviationregion may be formed to adjoin the p-type column layer (moreparticularly, a bottom surface of the p-type column layer).

In still another embodiment of the present disclosure, the depletionlayer alleviation region may be formed to be separated from the p-typecolumn layer (more particularly, the bottom surface of the p-type columnlayer).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a section view for explaining a configuration of asemiconductor device according to one embodiment of the presentdisclosure.

FIG. 2 is a perspective view of the semiconductor device illustratingsectional structures on two orthogonal cutting planes.

FIG. 3 is a plan view showing one example of a plan layout ofsemiconductor layers.

FIG. 4 is a plan view showing another example of a plan layout ofsemiconductor layers.

FIG. 5 is a section view for explaining an arrangement of a trap levelregion.

FIG. 6 is a section view for explaining an arrangement of a depletionlayer alleviation region.

FIG. 7 is an electric circuit diagram for explaining a configuration ofa power module in which the semiconductor device is used as a switchingelement.

FIGS. 8A through 8C are waveform diagrams showing different examples ofa current waveform when a parasitic diode is turned off from anon-state, FIG. 8A showing a current waveform in an instance whereneither the depletion layer alleviation region nor the trap level regionis provided (Comparative Example 1), FIG. 8B showing a current waveformin an instance where the depletion layer alleviation region is notprovided and the trap level region is provided (Comparative Example 2),FIG. 8C showing a current waveform in an instance where both thedepletion layer alleviation region and the trap level region areprovided (Example).

FIG. 9A is a section view showing the semiconductor device under amanufacturing process.

FIG. 9B is a section view showing the next step of the step shown inFIG. 9A.

FIG. 9C is a section view showing the next step of the step shown inFIG. 9B.

FIG. 9D is a section view showing the next step of the step shown inFIG. 9C.

FIG. 9E is a section view showing the next step of the step shown inFIG. 9D.

FIG. 9F is a section view showing the next step of the step shown inFIG. 9E.

DETAILED DESCRIPTION

Certain embodiments of the present disclosure will now be described indetail with reference to the accompanying drawings. FIG. 1 is a sectionview for explaining the configuration of a semiconductor deviceaccording to one embodiment of the present disclosure. FIG. 2 is aperspective view of the semiconductor device illustrating the sectionalstructures on two orthogonal cutting planes. The semiconductor device 1is an n-channel type MISFET having a super junction structure. Morespecifically, the semiconductor device 1 includes an n⁺-type drain layer10, an n-type base layer 12, a p-type column layer 14, a p-type baselayer 16, an n⁺-type source layer 18, a gate insulation film 20, a gateelectrode 22, a source electrode 26 (not shown in FIG. 2), a drainelectrode 28, a depletion layer alleviation region 30 and a trap levelregion 32. An interlayer insulation film 24 is arranged on the gateelectrode 22.

The n⁺-type drain layer 10 may be formed of an n⁺-type semiconductorsubstrate (e.g., a silicon substrate). The n⁺-type semiconductorsubstrate may be a semiconductor substrate obtained by causing crystalgrowth while doping an n-type impurity. As the n-type impurity, it ispossible to use P (phosphor), As (arsenic), Sb (antimony), etc. Then-type base layer 12 is a semiconductor layer doped with the n-typeimpurity. More specifically, the n-type base layer 12 may be an n-typeepitaxial layer which is caused to epitaxially grow while doping ann-type impurity. As the n-type impurity, it is possible to use thesubstances stated above.

The p-type column layer 14 and the p-type base layer 16 aresemiconductor layers doped with a p-type impurity. More specifically,the p-type column layer 14 and the p-type base layer 16 may besemiconductor layers formed by ion-implanting the p-type impurity intothe n-type base layer 12. As the p-type impurity, it is possible to useB (boron). Al (aluminum). Ga (gallium), etc. The p-type base layer 16 isselectively formed in a surface layer portion of the n-type base layer12 over a plurality of regions periodically arranged when thesemiconductor device 1 is seen in a plan view. For example, as shown ina plan view of FIG. 3, p-type base layers 16 having a rectangular shapemay be arranged in a staggering lattice pattern. Alternatively, as shownin a plan view of FIG. 4, p-type base layers 16 having a hexagonal shapemay be arranged in a staggered lattice pattern. A region including eachof the p-type base layers 16 and each of the n-type base layers 12arranged around the p-type base layers 16 makes up a cell 3. In otherwords, the semiconductor device 1 includes a plurality of cells 3arranged in a reticular shape when seen in a plan view.

The p-type column layer 14, when seen in a plan view, is formed in aregion existing in the p-type base layer 16 of each of the cells 3. Morespecifically, in the present embodiment, the p-type column layer 14 isformed in a substantially central region of the p-type base layer 16,when seen in a plan view, to have, e.g., a shape similar to the shape ofthe p-type base layer 16 (a substantially rectangular plan-view shape inthe layout of FIG. 3 or a hexagonal plan-view shape in the layout ofFIG. 3). The p-type column layer 14 is formed to adjoin the p-type baselayer 16. In the n-type base layer 12, the p-type column layer 14extends toward the n⁺-type drain layer 10 to reach a position deeperthan the p-type base layer 16. In other words, the p-type column layer14 is formed into a substantially columnar shape (a substantiallyrectangular columnar shape in the layout of FIG. 3 or a substantiallyhexagonal columnar shape in the layout of FIG. 4). A bottom surface 14 aof the p-type column layer 14 (a boundary surface of the p-type columnlayer 14 meeting with the n-type base layer 12) is arranged in aposition closer to the n⁺-type drain layer 10 than the center of thep-type column layer 14 in a thickness direction of the n-type base layer12. The bottom surface 14 a of the p-type column layer 14 is opposed toa surface of the n⁺-type drain layer 10 (an upper surface of the n⁺-typedrain layer 10 in FIGS. 1 and 2) with the n-type base layer 12 disposedtherebetween in the thickness direction. A side surface 14 b of thep-type column layer 14 (a boundary surface of the p-type column layer 14meeting with the n-type base layer 12) is opposed to a side surface 14 bof another adjoining p-type column layer 14 across the n-type base layer12.

The boundary surface between the p-type base layer 16 and the p-typecolumn layer 14 and the n-type base layer 12 is a p-n junction surfacemaking up a parasitic diode (body diode) 5. An n⁺-type source layer 18,when seen in a plan view, is formed in a region existing in the p-typebase layer 16 of each cell 3. In this region, the n⁺-type source layer18 is selectively formed in the surface layer portion of the p-type baselayer 16. The n⁺-type source layer 18 may be formed by selectivelyion-implanting an n-type impurity into the p-type base layer 16.Examples of the n-type impurity include the substances stated above. Then⁺-type source layer 18 is formed within the p-type base layer 16 to liein a position inwardly spaced apart by a specified distance from theboundary surface between the p-type base layer 16 and the n-type baselayer 12. In the surface layer regions of the semiconductor layersincluding the n-type base layer 12 and the p-type base layer 16,therefore, the surface layer portion of the p-type base layer 16 isinterposed between the n⁺-type source layer 18 and the n-type base layer12. The surface layer portion thus interposed provides a channel region17. In the present embodiment, the n⁺-type source layer 18 is formedinto an annular plan-view shape (a rectangular ring shape in the layoutof FIG. 3 or a hexagonal ring shape in the layout of FIG. 4). Then⁺-type source layer 18, when seen in a plan view, is formed in a regionstraddling the inside and the outside of the side surface 14 b of thep-type column layer 14. The channel region 17 has an annular plan-viewshape (a rectangular ring shape in the layout of FIG. 3 or a hexagonalring shape in the layout of FIG. 4) conforming to the shape of then⁺-type source layer 18.

The gate insulation film 20 may be made of, e.g., a silicon oxide film,a silicon nitride film, a silicon oxynitride film, hafnium oxide film,an alumina film or a tantalum oxide film. The gate insulation film 20 isformed to cover at least a surface of the p-type base layer 16 of thechannel region 17. In the present embodiment, the gate insulation film20 is formed to cover a portion of the n⁺-type source layer 18, thechannel region 17 and a surface of the n-type base layer 12. Moreprecisely, the gate insulation film 20 is formed in such a pattern as tohave an opening in a central region of the p-type base layer 16 of eachcell 3 and in an inner edge region of the n⁺-type source layer 18adjoining the central region.

The gate electrode 22 is formed to face the channel region 17 with thegate insulation film 20 interposed therebetween. The gate electrode 22may be made of, e.g., polysilicon whose resistance is reduced by dopingan impurity. In the present embodiment, the gate electrode 22 is formedsubstantially in the same pattern as the gate insulation film 20 tocover a surface of the gate insulation film 20. In other words, the gateelectrode 22 is arranged above the portion of the n⁺-type source layer18, the channel region 17 and the surface of the n-type base layer 12.More precisely, the gate electrode 22 is formed in such a pattern as tohave an opening in the central region of the p-type base layer 16 ofeach cell 3 and in the inner edge region of the n⁺-type source layer 18adjoining the central region. That is to say, the gate electrode 22 isformed to simultaneously control the plurality of cells 3.

The interlayer insulation film 24 is made of an insulating material,e.g., a silicon oxide film, a silicon nitride film or a TEOS(tetraethoxysilane) film. The interlayer insulation film 24 covers upperand side surfaces of the gate electrode 22. The interlayer insulationfilm 24 is formed in such a pattern as to have a contact hole 25 in thecentral region of the p-type base layer 16 of each cell 3 and in theinner edge region of the n⁺-type source layer 18 adjoining the centralregion. The source electrode 26 is made of aluminum or other metals. Thesource electrode 26 covers a surface of the interlayer insulation film24 and is formed to fill the contact hole 25 of each cell 3. Thus, thesource electrode 26 is ohmic-connected to the n⁺-type source layer 18.Accordingly, the source electrode 26 is parallel-connected to theplurality of cells 3 and is configured so that a total current flowingin the plurality of cells 3 can flow through the source electrode 26.The source electrode 26 is ohmic-connected to the p-type base layer 16of each cell 3 through the contact hole 25, thereby stabilizing anelectric potential of the p-type base layer 16.

The drain electrode 28 is made of aluminum or other metals. The drainelectrode 28 is formed on a rear surface of the n⁺-type drain layer 10(an opposite surface of the n⁺-type drain layer 10 from the n-type baselayer 12, i.e., a lower surface of the n⁺-type drain layer 10 in FIGS. 1and 2). Thus, the drain electrode 28 is parallel-connected to theplurality of cells 3 and is configured so that a total current flowingin the plurality of cells 3 can flow through the drain electrode 28. Ifa direct current source is connected between the source electrode 26 andthe drain electrode 28 with an electric potential of the drain electrode28 kept high and an electric potential of the source electrode 26 keptlow, a reverse bias is applied to the parasitic diode 5. At this time,if a control voltage lower than a predetermined threshold voltage isapplied to the gate electrode 22, no current path is formed between thedrain electrode 28 and the source electrode 26. That is to say, thesemiconductor device 1 is kept in an off-state. On the other hand, if acontrol voltage greater than the predetermined threshold voltage isapplied to the gate electrode 22, electrons are gathered on the surfaceof the channel region 17 and an inversion layer (channel) is formed onthe surface of the channel region 17. Thus, electric conduction is madebetween the n⁺-type source layer 18 and the n-type base layer 12. Inother words, there is formed a current path extending from the sourceelectrode 26 to the drain electrode 28 through the n⁺-type source layer18, the inversion layer of the channel region 17, the n-type base layer12 and the n⁺-type drain layer 10 in the named order. Thus, thesemiconductor device 1 is kept in an on-state. In FIG. 1, the currentpath corresponding to the central cell 3 is indicated by a broken-linearrow.

When the semiconductor device 1 is applied to an inverter circuit fordriving an inductive load such as an electric motor, it is sometimes thecase that the source electrode 26 becomes higher in electric potentialthan the drain electrode 28 and the parasitic diode 5 gets turned on,thereby allowing a current to flow through the parasitic diode 5.Thereafter, if the source electrode 26 becomes lower in electricpotential than the drain electrode 28, the parasitic diode 5 goes into areverse bias state and gets turned off. At this turning-off time, adepletion layer is expanded from the p-n junction portion of theparasitic diode 5. Thus, carriers (positive holes) within the p-typebase layer 16 and the p-type column layer 14 migrate toward the sourceelectrode 26, and carriers (electrons) within the n-type base layer 12migrate toward the drain electrode 28. Due to this migration of thecarriers, a current flows in an opposite direction to the currentflowing direction when the parasitic diode 5 is in an on-state. Thiscurrent is called a reverse recovery current. The reverse recoverycurrent is initially increased and then decreased. The time required foran intensity of the reverse recovery current to decrease to 10% of amaximum value after a forward current of the diode becomes zero iscalled a reverse recovery time. If a change of the reverse recoverycurrent (di/dt) is great, vibration (ringing) may sometimes occur untilthe current converges to zero. These recovery characteristics are calledhard recovery which may cause noise and erroneous operations.

The trap level region 32 assists in shortening the reverse recoverytime. The depletion layer alleviation region 30 helps alleviate the hardrecovery.

FIG. 5 is a section view for explaining an arrangement of the trap levelregion 32. The trap level region 32 is a region formed by irradiatingbaryons at the side of the drain layer 10. In the trap level region 32,there exist a lot of recombination centers where the carriers aretrapped, recombined and extinguished. This makes it possible to rapidlyextinguish the carriers when the parasitic diode 5 is turned off.Accordingly, it is possible to reduce the reverse recovery time and thereverse recovery current.

The trap level region 32 is locally formed to expand in a thin thickness(e.g., about 1 μm to 3 μm) at a predetermined depth position from therear surface of the drain layer 10 (the boundary surface of the drainlayer 10 meeting with the drain electrode 28). The trap level region 32may adjoin the p-type column layer 14 as indicated by reference symbol32A or may be positioned between the bottom surface 14 a of the p-typecolumn layer 14 and the drain layer 10 without adjoining the p-typecolumn layer 14 as indicated by reference symbols 32B and 32C. Thereference symbol 32B indicates an instance where a thickness directioncenter position of the trap level region 32 is closer to the bottomsurface 14 a of the p-type column layer 14 than a middle positionbetween the bottom surface 14 a of the p-type column layer 14 and thesurface of the drain layer 10. The reference symbol 32C indicates aninstance where the thickness direction center position of the trap levelregion 32 is closer to the drain layer 10 than the middle positionbetween the bottom surface 14 a of the p-type column layer 14 and thesurface of the drain layer 10. It is effective in shortening the reverserecovery time if the trap level region 32 is positioned close to thebottom surface 14 a of the p-type column layer 14, while it is effectivein reducing the drain-source leak current if the trap level region 32 ispositioned far away from the bottom surface 14 a of the p-type columnlayer 14. In order to reduce the reverse recovery time and thedrain-source leak current, in some embodiments the thickness directioncenter position of the trap level region 32 lies within a range of 5 μmto 10 μm from the bottom surface 14 a of the p-type column layer 14toward the drain layer 10. This makes it possible to reduce the reverserecovery time to 80 nanoseconds or less and to reduce the drain-sourceleak current to several μA or less.

When forming the trap level region 32, it is possible to adoptirradiation of baryons such as protons, ³He⁺⁺ and ⁴He⁺⁺. Among them, thehelium nuclei (³He⁺⁺ or ⁴He⁺⁺) having a large mass are preferred becausethe helium nuclei are capable of narrowing a thickness-directiondistribution band of the recombination centers and capable of locallydistributing the recombination centers within a narrow range in thethickness direction.

FIG. 6 is a section view for explaining an arrangement of the depletionlayer alleviation region 30. The depletion layer alleviation region 30is a region formed by irradiating baryons at the side of the drain layer10 and converting the baryons to donors though heat treatment. Thebaryons converted to donors restrain a depletion layer from expandingfrom the p-n junction portion when the parasitic diode 5 is turned off.As a result, an expansion speed of the depletion layer is reduced, whichmakes it possible to reduce a changing speed of the reverse recoverycurrent and, consequently, to alleviate the hard recovery.

The depletion layer alleviation region 30 is formed to expand in athickness direction (with a thickness of, e.g., about 5 μm to 10 μm,which is thicker than the trap level region 32) at a predetermined depthposition from the rear surface of the drain layer 10 (the boundarysurface of the drain layer 10 meeting with the drain electrode 28). Thedepletion layer alleviation region 30 may be contiguous to the p-typecolumn layer 14 as indicated by reference symbols 30A and 30B or may notbe contiguous to the p-type column layer 14 as indicated by referencesymbol 30C. In addition, the depletion layer alleviation region 30 mayhave an area overlapping with the p-type column layer 14 as indicated bythe reference symbol 30B. Alternatively, the entire depletion layeralleviation region 30 may be positioned between the bottom surface 14 aof the p-type column layer 14 and the surface of the drain layer 10 withno area overlapping with the p-type column layer 14 as indicated by thereference symbols 30A and 30C. Since the depletion layer alleviationregion 30 is a region including donors, in some embodiments, in ordernot to impair the function of the p-type column layer 14, the areaoverlapping with the p-type column layer 14 is made to be as narrow aspossible. With a view to alleviate the expansion of the depletion layer,in some embodiments the depletion layer alleviation region 30 should beclose to the p-type column layer 14. It is therefore, in someembodiments, preferable to select the arrangement of the depletion layeralleviation region 30 so that, as indicated by the reference symbol 30A,an upper edge of the depletion layer alleviation region 30 besubstantially flushed with the bottom surface 14 a of the p-type columnlayer 14.

When forming the depletion layer alleviation region 30, it is possibleto adopt irradiation of baryons such as protons, ³He⁺⁺ and ⁴He⁺⁺. Amongthem, the protons having a small mass can be introduced to have a widedistribution in the thickness direction and, therefore, are suitable foruse in forming a thick depletion layer alleviation region 30. Theprotons can be converted to donors by heat treatment at a relatively lowtemperature (e.g., at 350 to 450 degrees Celsius). Therefore, theirradiation of the protons and the conversion to donors thereof (theheat treatment) can be performed, e.g., before and after the formationof the drain electrode 28 or the like. Accordingly, use of the protonshelps increase the degree of freedom of the process.

The arrangement of the depletion layer alleviation region 30 shown inFIG. 6 and the arrangement of the trap level region 32 shown in FIG. 5can be combined arbitrarily.

FIG. 7 is an electric circuit diagram for explaining the configurationof a power module in which the semiconductor device 1 is used as aswitching element. The power module 40 makes up a motor drive circuit(an inverter circuit) for driving a three-phase alternating currentelectric motor 60 (one example of inductive loads) with the electricpower supplied from a battery 50 as a direct current source. Theelectric motor 60 can be used in, e.g., driving a fan, a compressor orthe like.

The power module 40 includes a U-phase serial circuit 41, a V-phaseserial circuit 42 and a W-phase serial circuit 43 corresponding to aU-phase, a V-phase and a W-phase of the electric motor 60, respectively.These serial circuits 41, 42 and 43 are parallel-connected to thecurrent source 50. The serial circuits 41, 42 and 43 of these threephases may be accommodated in a single housing. The U-phase serialcircuit 41 is formed by serially connecting a high-side switchingelement Su1 and a low-side switching element Su2. A connection point 44between the switching elements Su1 and Su2 is connected to a U-phasecoil 61 of the electric motor 60. Similarly, the V-phase serial circuit42 is formed by serially connecting a high-side switching element Su3and a low-side switching element Su4. A connection point 45 between theswitching elements Su3 and Su4 is connected to a V-phase coil 62 of theelectric motor 60. Likewise, the W-phase serial circuit 43 is formed byserially connecting a high-side switching element Su5 and a low-sideswitching element Su6. A connection point 46 between the switchingelements Su5 and Su6 is connected to a W-phase coil 63 of the electricmotor 60.

Each of the switching elements Su1 through Su6 is formed of thesemiconductor device 1 set forth above. The parasitic diode 5 of each ofthe switching elements Su1 through Su6 is reversely connected to thebattery 50. A control signal from a controller 70 is inputted to a gateof each of the switching elements Su1 through Su6. The control signalmay be a PWM (pulse width modulation) signal having a duty ratiocorresponding to an electric power to be supplied to the electric motor60.

The controller 70 serves as an on-off control for the switching elementsSu1 through Su6 depending on a rotation angle (an electric angle) of arotor of the electric motor 60. Thus, the rotor of the electric motor 60generates torque. Coils 61 through 63 of the electric motor 60 generatea counter electromotive force upon interruption of the electric power.Due to the counter electromotive force, the parasitic diode 5 of each ofthe switching elements Su1 through Su6 is turned on, thereby generatinga time period during which a current flows through the parasitic diode5. In other words, the parasitic diode 5 is turned on and off while theelectric motor 60 is driven. For instance, in any one of the serialcircuits 41, 42 and 43, there may be a case where the high-sideswitching element is in an on-state and the parasitic diode 5 of thelow-side switching element is also in an on-state. When the parasiticdiode 5 of the low-side switching element is turned off with thehigh-side switching element kept in the on-state, a through-currentflows due to a reverse recovery current.

FIGS. 8A through 8C are waveform diagrams showing different examples ofwaveforms of a current flowing between the source electrode 26 and thedrain electrode 28 when the parasitic diode 5 is turned off from theon-state. FIG. 8A shows a current waveform in an instance where neitherthe depletion layer alleviation region 30 nor the trap level region 32is provided (Comparative Example 1). FIG. 8B shows a current waveform inan instance where the depletion layer alleviation region 30 is notprovided and the trap level region 32 is provided (Comparative Example2). FIG. 8C shows a current waveform in an instance where both thedepletion layer alleviation region 30 and the trap level region 32 areprovided (Example).

During time period P1, the parasitic diode 5 remains conductive suchthat a current flows in a forward direction of the parasitic diode 5. Ifthe source electrode 26 becomes lower in electric potential than thedrain electrode 28 at time t1, the parasitic diode 5 comes into areverse bias state and gets turned off. At this turning-off time, adepletion layer is expanded from the p-n junction portion of theparasitic diode 5. Thus, the carriers (positive holes) within the p-typebase layer 16 and the p-type column layer 14 migrate toward the sourceelectrode 26, and the carriers (electrons) within the n-type base layer12 migrate toward the drain electrode 28. Due to this migration of thecarriers, a current (a reverse recovery current) flows in an oppositedirection to the current flowing direction when the parasitic diode 5 isin an off-state. The reverse recovery current is initially increased (inabsolute value) and then decreased. As stated earlier, the time requiredfor the intensity of the reverse recovery current to decrease to 10% ofthe maximum value after the forward current of the diode becomes zero isa reverse recovery time trr.

The trap level region 32 reduces the number of migrating carriers, whichhelps reduce the reverse recovery current and the reverse recovery timetrr. For that reason, the reverse recovery time trr is shorter inComparative Example 2 shown in FIG. 8B and in the Example shown in FIG.8C than in Comparative Example 1 shown in FIG. 8A. More specifically,the reverse recovery time trr in Comparative Example 1 shown in FIG. 8Ais about 160 nanoseconds, the reverse recovery time trr in ComparativeExample 2 shown in FIG. 8B is about 80 nanoseconds, and the reverserecovery time trr in the Example shown in FIG. 8C is about 90nanoseconds.

The depletion layer alleviation region 30 reduces an expansion speed ofthe depletion layer, thereby reducing the changing speed of the reverserecovery current (di/dt). More specifically, the rate of change of thereverse recovery current is reduced after the reverse recovery currentis turned to reduction (in absolute value). In other words, the rate ofchange of the reverse recovery current after the reverse recoverycurrent is turned to reduction is smaller in the Example shown in FIG.8C than in Comparative Example 1 shown in FIG. 8A and ComparativeExample 2 shown in FIG. 8B. Thus, the current rapidly converges to zeroin the Example shown in FIG. 8C.

In the Example shown in FIG. 8C, the reverse recovery current smoothlyconverges to zero as set forth above, which makes it possible to solvethe problem of hard recovery. In other words, it is possible to reducethe ringing (vibration of the reverse recovery current) as indicated byreference symbols A and B in FIGS. 8A and 8B and the noise generated bythe sharp change of the current. As a result of the reduction of noise,it becomes possible to reduce the erroneous operation of the controller70 and to stably control the electric motor 60.

FIGS. 9A through 9F are section views illustrating a process ofmanufacturing the semiconductor device 1. Referring first to FIG. 9A,the n-type base layer 12 greater in resistance (lower in impurityconcentration) than the n⁺-type drain layer 10 is formed on the n⁺-typedrain layer 10 made from, e.g., an n⁺-type silicon substrate, byepitaxial growth which is performed while doping an n-type impurity.Then, as illustrated in FIG. 9B, p-type impurity ions are selectivelyimplanted into the surface layer portion of the n-type base layer 12with a relatively low level of energy, thereby forming the p-type baselayer 16. P-type impurity ions are selectively implanted into apredetermined plan-view region within the p-type base layer 16 with arelatively high level of energy, consequently forming the p-type columnlayer 14 that extends to a region deeper than the p-type base layer 16.N-type impurity ions are selectively implanted into an annular region ofspecified width having an outer edge portion in a position inwardlydeviated by a specified distance from the outer edge of the p-type baselayer 16 in the p-type base layer 16 as seen in a plan view, therebyforming the n⁺-type source layer 18.

Next, as illustrated in FIG. 9C, the gate insulation film 20 is formedto cover the surfaces of the n-type base layer 12 and the p-type baselayer 16 (the surfaces of semiconductor crystals). The gate insulationfilm 20 may be formed by thermally oxidizing the semiconductor crystalsurfaces. The gate electrode 22 is formed on the gate insulation film20. The formation of the gate electrode 22 may be performed by, e.g.,forming a polysilicon film, the resistance of which is reduced byaddition of an impurity, on the whole surface and then selectivelyetching the polysilicon film by photolithography. In the etching step,the gate electrode 22 and the gate insulation film 20 may be formed withthe same pattern by simultaneously patterning the gate electrode 22 andthe gate insulation film 20. Then, the interlayer insulation film 24 isformed to cover the gate electrode 22. The contact hole 25 is formed inthe interlayer insulation film 24 by photolithography.

Next, as illustrated in FIG. 9D, the source electrode 26 is formed onthe interlayer insulation film 24, and the drain electrode 28 is formedon the rear surface of the drain layer 10. Thereafter, if necessary,heat treatment is performed to form an ohmic junction through analloying process. The formation of the drain electrode 28 may beperformed by, e.g., a process in which a base layer (an ohmic junctionlayer) of the drain electrode 28 is formed by vapor deposition of goldand antimony. In this case, the heat treatment for the ohmic junctionmay be performed at a temperature of, e.g., about 420 degrees Celsius.An Au—Si eutectic can be formed by the heat treatment.

Subsequently, as illustrated in FIG. 9E, first baryon irradiation isperformed at the side of the drain layer 10. As the baryons (firstbaryons) irradiated at this time, it is possible to use baryons having arelatively small mass, e.g., protons. Thereafter, low-temperature heattreatment (low-temperature annealing) is carried out. Thus, theirradiated baryons are converted to donors. When protons are selected asthe baryons, the introduced protons can be converted to donors by, e.g.,performing heat treatment at about 350 to 450 degrees Celsius (e.g., 360degrees Celsius) for about 30 to 90 minutes (e.g., 60 minutes). If thealloying of the source electrode 26 and the drain electrode 28 issimultaneously performed by heat treatment at this time, the heattreatment to be carried out after the formation of the source electrode26 and the drain electrode 28 may be omitted.

In this manner, the depletion layer alleviation region 30 is formed bythe irradiation of the first baryons and the subsequent heat treatment.If the level of energy during the irradiation of the first baryons ishigh, the range of the first baryons becomes longer and, therefore, thedepletion layer alleviation region 30 is formed in a position distantfrom the rear surface of the drain layer 10. If the level of energyduring the irradiation of the first baryons is low, the range of thefirst baryons becomes shorter and, therefore, the depletion layeralleviation region 30 is formed in a position close to the rear surfaceof the drain layer 10. In view of this, the irradiation energy of thefirst baryons is set depending on a placement of the depletion layeralleviation region 30. The irradiation energy of the first baryons isset (to, e.g., about 8 MeV) such that at least a portion of thedepletion layer alleviation region 30 is positioned between the bottomsurface 14 a of the p-type column layer 14 and the drain layer 10. Thedose amount of the first baryons (e.g., protons) is in some embodimentsset to, e.g., about 5×10¹³ baryons/cm² to 1×10¹⁴ baryons/cm².

Next, as illustrated in FIG. 9F, second baryon irradiation is performedat the side of the drain layer 10. As the baryons (second baryons) areirradiated at this time, it is possible to use baryons having arelatively large mass, e.g., helium nuclei (³He⁺⁺ or ⁴He⁺⁺). Thereafter,low-temperature heat treatment (low-temperature annealing) is carriedout, whereby the irradiated second baryons are activated. When heliumnuclei (³He⁺⁺ or ⁴He⁺⁺) are selected as the second baryons, theintroduced helium nuclei can be activated by, e.g., performing heattreatment at about 320 to 380 degrees Celsius (e.g., 350 degreesCelsius) for about 30 to 120 minutes (e.g., 60 minutes).

In this manner, the trap level region 32 is formed. If the level ofenergy during the irradiation of the second baryons is high, the rangeof the second baryons becomes longer and, therefore, the trap levelregion 32 is formed in a position distant from the rear surface of thedrain layer 10. If the level of energy during the irradiation of thesecond baryons is low, the range of the second baryons becomes shorterand, therefore, the trap level region 32 is formed in a position closeto the rear surface of the drain layer 10. In view of this, theirradiation energy of the second baryons is set depending on theplacement of the trap level region 32. For example, the irradiationenergy of the second baryons is set (to, e.g., about 23 MeV) such thatthe trap level region 32 is positioned between the bottom surface 14 aof the p-type column layer 14 and the drain layer 10. The dose amount ofthe second baryons is in some embodiments set to, e.g., about 5×10¹⁰baryons/cm² to 5×10¹² baryons/cm².

Thereafter, if necessary, a surface layer of the drain electrode 28 isformed on a base layer (e.g., the gold-antimony layer) of the drainelectrode 28. The surface layer may be a laminated film formed bylaminating Ti/Ni/Au/Ag layers one above another on the base layer. Thelaminated film can be continuously formed by a sputtering method. Themelting points of the metals of the respective layer making up thelaminated film are higher than the temperature of the heat treatment forthe conversion to donors of the protons and for the activation of thehelium nuclei. Thus, from the viewpoint of physics, no problem is posedeven if the formation of the laminated film is performed prior to theheat treatment for the conversion to donors of the protons and for theactivation of the helium nuclei. In the present embodiment, however, theformation of the surface layer is performed after the heat treatment forthe conversion to donors of the protons and for the activation of thehelium nuclei so that a thermal history should not remain in the surfacelayer formed of the laminated film.

The semiconductor device 1 shown in FIG. 1 and other figures ismanufactured in the manner described above.

In the semiconductor device 1 of the present embodiment described above,the p-type column layer 14 extends from the p-type base layer 16 towardthe n⁺-type drain layer 10 and makes up a MISFET having a super junctionstructure. This makes it possible to provide a switching element havinga low on-resistance and an increased switching speed. In thesemiconductor device 1, the depletion layer alleviation region 30 isarranged between the p-type column layer 14 and the n⁺-type drain layer10 within the p-type base layer 16. The depletion layer alleviationregion 30 includes baryons converted to donors. The depletion layeralleviation region 30 restrains a depletion layer from expanding towardthe n⁺-type drain layer 10, thereby reducing the expansion speed of thedepletion layer when the parasitic diode 5 is turned off. This reducesthe changing speed of a reverse recovery current and, therefore,improves the recovery characteristics. Accordingly, it is possible toprovide a MISFET which is good in on-resistance and switching speed andsuperior in recovery characteristics.

In the semiconductor device 1 of the present embodiment, the trap levelregion 32 containing baryons for formation of a trap level is formedwithin the n-type base layer 12. The trap level region 32 serves to trapcarriers (electrons) migrating within the n-type base layer 12, therebyreducing the reverse recovery current. Accordingly, the depletion layeralleviation region 30 can improve the recovery characteristics while thetrap level region 32 can shorten the reverse recovery time trr.

In the semiconductor device 1 of the present embodiment, the thicknessof the depletion layer alleviation region 30 is greater than thethickness of the trap level region 32. Since the trap level is locallydistributed within a limited extent, it is possible to shorten thereverse recovery time while reducing the leak current. Inasmuch as thedepletion layer alleviation region 30 is formed within the n-type baselayer 12 over a relatively wide range in the area between the p-typecolumn layer 14 and the n⁺-type drain layer 10, it is possible toeffectively reduce the expansion speed of the depletion layer and,hence, to effectively improve the recovery characteristics.

When protons are selected as the first baryons, the conversion to donorsthereof can be performed through heat treatment at a relatively lowtemperature (e.g., 350 to 450 degrees Celsius). For that reason, theheat treatment can be carried out after the formation of the sourceelectrode 26 and/or the drain electrode 28. Accordingly, the degree offreedom of the process grows higher. If the boundary of the depletionlayer alleviation region 30 coincides with or remains close (by adistance of, e.g., 5 μm or less) to the boundary of the p-type columnlayer 14, the speed at which the depletion layer expands from the p-typecolumn layer 14 can be reduced without eroding the p-type column layer14. This makes it possible to improve the recovery characteristicswithout impairing the effect provided by the super junction structure.

In the power module 40 in which the semiconductor device 1 having thesuper junction structure is used as the high-side switching elementsSu1. Su3 and Su5 and the low-side switching elements Su2, Su4 and Su6,the power loss in the switching elements Su1 through Su6 remains low,which leads to increased power supply efficiency. Since the hardrecovery occurring when the parasitic diodes 5 of the switching elementsSu1 through Su6 are turned off is restrained, it is possible to reducethe erroneous operation of the controller 70 for supplying a controlsignal to the switching elements Su1 through Su6.

While one embodiment of the present disclosure has been described above,the present disclosure can be embodied in many different forms. Forexample, although protons are used as the first baryons and heliumnuclei are used as the second baryons in the embodiment described above,the combination of the baryons is not limited thereto. Morespecifically, protons, ³He⁺⁺ and ⁴He⁺⁺ presented as examples of thefirst baryons and protons, ³He⁺⁺ and ⁴He⁺⁺ presented as examples of thesecond baryons can be used in arbitrary combinations.

Although the formation of the depletion layer alleviation region 30 andthe formation of the trap level region 32 through the irradiation of thefirst baryons and the second baryons are performed after the electrodeformation step (see FIG. 9D) in the embodiment described above, one orboth of them may be performed prior to the electrode formation step. Inthis case, however, the temperature of heat treatment for the electrodeformation (alloying process for the formation of ohmic contacts) needsto be lower than the temperature of heat treatment for the formation ofthe depletion layer alleviation region 30 and/or the trap level region32.

While the structure and manufacturing method of the semiconductor device1 including both the depletion layer alleviation region 30 and the traplevel region 32 has been presented in the embodiment described above,the trap level region 32 may be omitted as far as only hard recoveryneeds to be improved. While the power module 40 including the serialcircuits of three phases 41 through 43 for driving the three-phasealternating current electric motor 60 has been presented in theembodiment described above, a serial circuit of one phase (a serialcircuit of a high-side switching element and a low-side switchingelement) may be accommodated in a single housing as a unit of a powermodule. In this case, a motor drive circuit for the three-phasealternating current electric motor 60 can be formed byparallel-connecting power modules of three units. If the electric motorto be driven is not a three-phase motor, a motor drive circuit can beformed by parallel-connecting power modules whose unit numbercorresponds to the phase number of the electric motor.

While the electric motor has been presented as one example of inductiveloads in the embodiment described above, the present disclosure may beapplied to a power module for driving other inductive loads such as arelay and a solenoid. Moreover, the present disclosure may be appliednot only to the power module for driving inductive loads but also to adirect inverter for driving a backlight of a liquid crystal televisionset, a power conditioner for photovoltaic power generation, and soforth. It goes without saying that the present semiconductor device canbe applied not only to the inverter circuit but also to a circuit havinga fast-driven switching element, e.g., a converter circuit.

While the p-type base layer 16 and other layers are formed into arectangular plan-view shape (see FIG. 3) or a hexagonal plan-view shape(see FIG. 4) in the embodiment described above, the plan-view shape ofthe p-type base layer 16 and other layers is not limited thereto but maybe circular, elliptical, pentagonal or other polygonal shape. Inaddition, the present disclosure can be modified to many differentdesigns without departing from the scope of the disclosure defined inthe claims.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the disclosures. Indeed, the novel semiconductor device andmethod of manufacturing the same described herein may be embodied in avariety of other forms; furthermore, various omissions, substitutionsand changes in the form of the embodiments described herein may be madewithout departing from the spirit of the disclosures. The accompanyingclaims and their equivalents are intended to cover such forms ormodifications as would fall within the scope and spirit of thedisclosures.

What is claimed is:
 1. A method of manufacturing a semiconductor device,comprising: forming an n-type base layer on an n-type drain layer;partially forming a p-type base layer in a surface layer portion of then-type base layer; partially forming an n-type source layer in a surfacelayer portion of the p-type base layer; forming a p-type column layerformed within the n-type base layer to extend from the p-type base layertoward the n-type drain layer; forming a gate insulation film on asurface of the p-type base layer between the n-type source layer and then-type base layer; forming a gate electrode on the gate insulation filmto face, through the gate insulation film, the surface of the p-typebase layer between the n-type source layer and the n-type base layer;forming a source electrode electrically connected to the n-type sourcelayer; forming a drain electrode electrically connected to the n-typedrain layer; and forming a depletion layer alleviation region arrangedbetween the p-type column layer and the n-type drain layer so as to bewithin the n-type base layer by irradiating first baryons at a rearsurface side of the n-type drain layer and then converting the firstbaryons to donors through heat treatment at a predetermined temperature;forming a trap level region including second baryons locally formedwithin the depletion layer alleviation region, the second baryons beingheavier than the first baryons.
 2. The method of claim 1, furthercomprising: forming the trap level region within the n-type base layerby irradiating the second baryons at the rear surface side of the n-typedrain layer.
 3. The method of claim 2, wherein the trap level region islocally formed within the depletion layer alleviation region.
 4. Themethod of claim 2, wherein the depletion layer alleviation region andthe trap level region are formed such that the depletion layeralleviation region has a thickness greater than a thickness of the traplevel region.
 5. The method of claim 2, wherein the second baryonscomprise one member selected from the group consisting of protons, ³He⁺⁺and ⁴He⁺⁺.
 6. The method of claim 1, wherein the first baryons compriseone member selected from the group consisting of protons, ³He⁺⁺ and⁴He⁺⁺.
 7. The method of claim 1, wherein the first baryons compriseprotons.
 8. The method of claim 1, wherein the depletion layeralleviation region is formed to have an area overlapping with the p-typecolumn layer.
 9. The method of claim 1, wherein the depletion layeralleviation region is formed not to have an area overlapping with thep-type column layer.
 10. The method of claim 1, wherein the depletionlayer alleviation region is formed to be adjacent to the p-type columnlayer.
 11. The method of claim 1, wherein the depletion layeralleviation region is formed to be separated from the p-type columnlayer.
 12. A method of manufacturing a power module, comprising: forminga high-side switching element formed of a semiconductor devicemanufactured by the method of claim 1; and forming a low-side switchingelement formed of a semiconductor device manufactured by the method ofclaim 1, the high-side switching element and the low-side switchingelement being serially connected to each other.
 13. The method of claim12, wherein the high-side switching element comprises high-sideswitching elements and the low-side switching element includes low-sideswitching elements, the method further comprising: forming a threeserial circuits, each including one of the high-side switching elements,and one of the low-side switching elements that are serially connectedto each other, the three serial circuits being parallel-connected to oneanother.
 14. The method of claim 12, wherein the power module is used tosupply a drive current to an alternating current electric motor.